Supergene (geology)

Metals that have been leached from the oxidized ore are carried downward by percolating groundwater, and react with hypogene sulfides at the supergene-hypogene boundary.

[5] For example, groundwater commonly interacts with pyrite (FeS2) to form an oxidized iron (FeO(OH)) and sulfuric acid (H2SO4), portrayed in the idealized chemical reaction below (intermediate steps omitted): An intermediate in this process is ferric sulfate (Fe2(SO4)3), which oxidizes pyrite and other sulfide minerals.

[7] Copper ions that move down into this reducing environment form a zone of supergene sulfide enrichment.

[3] Covellite (CuS), chalcocite (Cu2S) and native copper (Cu) are stable in these conditions[7] and they are characteristic of the enriched zone.

[5] Chalcopyrite CuFeS2 (primary) readily alters to the secondary minerals bornite Cu5FeS4, covellite CuS and brochantite Cu4SO4(OH)6.

[5] If the original deposits contain arsenic and phosphorus bearing minerals, secondary arsenates and phosphates will be formed.